Method for selection of a strategy for re-routing of circuits in a communication network and network with said method

ABSTRACT

A method is described for identification in a communication network of a sequence of circuit movement steps therein to change with the movement of a single circuit at a time from an initial routing in which are satisfied a certain number of service demands to a feasible target routing that satisfies the same service demands with better utilization of the network resources and that is identified as the one which best approximates or identifies a new predetermined desired target routing. In accordance with the method the steps are made of calculating for each demand still to be processed a replacement circuit reducing the cost difference with the circuit that satisfies the same demand in the desired target routing, choosing from among all the calculated replacement circuits the one having less cost and replacing it, marking the corresponding demand as processed, and repeating the steps from the beginning until all the demands have been processed. Then the sequence with which the circuits were replaced is used as a sequence of steps in moving of circuits for re-routing the network.

The present invention relates to a method for selection of a strategyfor re-routing of circuits in a network.

When a connectivity service is required on a transmission network thenetwork resources are allocated with an optimization criterion chosen onthe basis of various parameters.

Allocation of resources is commonly called ‘circuit routing’.

Although in the allocation of resources the optimum is required tosupply the required services in a data transport network (mainly interms of band consumption but also with reference to other parameters),in the long term these resources tend for various reasons to be used ina more or less inefficient manner. In other words, during the life of anetwork many variable circumstances can alter the parameters of choiceso that the routing is no longer optimal or, at the worst, can become nolonger acceptable. Examples of reasons which might make the routing nolonger satisfactory as when it was calculated can be various, such as:

-   -   Some circuits were allocated first and, because of this, other        subsequent circuits were necessarily routed in a not optimal        manner on the remaining resources and then the first series of        circuits is cancelled so that the not-optimal circuits on the        remaining resources could have room for improvement. In a more        complicated manner, this is what happens regularly when a        network is made to operate for a long time;    -   New network resources (nodes and fibers) are installed after        many circuits are routed and these circuits could also be        improved by using the available resources;    -   Some resources need to be freed for maintenance reasons; and    -   New routing algorithms are developed while optimizing them with        different criteria.

The change in conditions can require rerouting of a large number ofcircuits.

Now the simplest answer for overall rerouting of the circuits could beto cancel everything and reconfigure the network from zero.Unfortunately, such a procedure would imply a long period out ofcommission for a large series of circuits. Furthermore, there is a notnegligible risk that something might go wrong during the rerouting stepand in this case an unpredictable out-of-commission situation couldinvolve many circuits.

In a certain sense, this is the same problem found with files memorizedon a hard disk; after a certain time the files on the hard disk tend tobecome badly fragmented and, to return to better efficiency, adefragmentation operation is necessary and could be long and not withoutrisks.

Defragmentation in a telecommunications network can be more difficult torealize than on a hard disk since the network transports importantservices for a large number of customers who would not accept aninterruption of the service during defragmentation.

One solution to the problem could be to move one circuit at a time so asto minimize the risks and out-of-commission periods in the network.Although this has been proposed theoretically, the problem of applyingthis solution practically remains unsolved because a new theoreticallycalculated routing cannot always be obtained in reality. Indeed, thecircuits allocated can obstruct each other and not leave enough spacefor handling. In this case there could even be some movement sequencesallowing the right movement as in a game of patience but the problem isto identify them. At the worst, it could also happen that there may notbe any sequence allowing the new routing exactly as calculated.

The general purpose of the present invention is to remedy the abovementioned shortcomings by making available a method for comparing theallocation of resources and services on a network with a desired newallocation, estimating the feasibility of the desired allocation,calculating a feasible allocation which would best approximate the onedesired and reorganizing the new routings calculated to arrange them inan order allowing moving the circuits one at a time to limit the risksand minimize the traffic interruptions. In other words, the purpose ofthe present invention, given an old and a new routing, is to provide aprocedure for finding a feasible routing which would best approximatethe new routing and a feasible order for moving the circuits one at atime from the old routing to the new feasible routing.

But the present invention is not addressed to how to calculate thedesired routing (which can still be calculated by any known method) andassumes that the new desired routing is already known and calculatedwith any criterion or even designed manually.

The present invention is quite applicable to networks using protectiondiagrams similar to the well known 50 msec SNCP for SDH networks. Inthis case, indeed, the out-of-commission time of each circuit because ofits movement is equal to only the protection circuit intervention time.

In view of this purpose it was sought to provide in accordance with thepresent invention a method for identification of a sequence of circuitmovement steps in a communication network for re-routing of the networkto change with the movement of a single circuit at a time from aninitial routing made up of a series of n circuits CA_(i) which satisfycorresponding demands R_(i) (with i=1, . . . , n) to a feasible targetrouting made up of a new series of circuits CI_(i) which continue tosatisfy the demands R_(i) and which is identified as the one which bestapproximates or identifies a desired target routing and which is made upof a series of n circuits CT_(i) which still satisfy the demands R_(i)(with i=1, . . . , n) and with the method comprising the steps, startingfrom a feasible routing which is set to be equal to the initial routing,of:

-   (a) Calculating for each demand R_(i) still to be processed a    replacement circuit CI_(i) which would reduce the cost difference    with the circuit CT_(i), would satisfy the demand R_(i) and is in    the desired target routing;-   (b) Choosing from among all the replacement circuits CI_(i)    calculate under step (a) the one which has least cost and replacing    with it the circuit which satisfies the corresponding demand R_(i)    in the present feasible routing;-   (c) Marking as processed the corresponding demand R_(i) which is    satisfied by the replaced circuit CI_(i);-   (d) Repeating steps (a) to (c) until all the demands R_(i) have been    processed; and-   (e) Taking as the sequence of steps for re-routing the network the    sequence with which the circuits in the feasible routing have been    replaced until the feasible target routing is achieved.

To clarify the explanation of the innovative principles of the presentinvention and its advantages compared with the prior art there isdescribed below with the aid of the annexed drawings a possibleembodiment thereof by way of non-limiting example applying saidprinciples. In the drawings:

FIG. 1 shows an overall view of the re-routing process of a networkapplying the method of the present invention;

FIG. 2 shows a flow chart of the method of the present invention; and

FIGS. 3 to 8 show the steps of the method applied to an example of asimple network.

With reference to the figures, FIG. 1 shows diagrammatically the overallview of a network re-routing process. When re-routing is desired (thedecision on this is beyond the purposes of the present invention) the‘Network Management System’ or ‘Control Plan’ of the network sends tothe Network Simulator the ‘Network Config’ updating and the ‘ActualRouting’ i.e. the routing in use in the network at the time. Ifnecessary, other input parameters (‘param’) provided by a human operatoralso reach the simulator. By applying known methods and procedures notdescribed in detail, the simulator calculates the ‘target routing’ i.e.the routing that would be desired to optimize the network in accordancewith the predetermined parameters.

At this point the method of the present invention is applied and,starting from the actual routing and desired routing, provides afeasible target routing (FTR) which best approximates the desired targetrouting and a re-routing sequence permitting achievement thereof bymoving a single circuit at a time.

In accordance with another view, a sequence of routings is thus definedstarting from the actual routing and reaches the feasible target routingwith each routing in the sequence differentiated by a single circuitfrom the routing which precedes it and follows it in the sequence.

If required, after the new network configure has been accepted by theoperator the re-routing sequence is sent to the network manager who willimplement it to realize the safe re-routing of the network.

The method in accordance with the present invention allows obtainingfirst of all the best feasible series of routing circuits i.e. obtainingthe desired target routing if possible or the best approximation of thetarget routing it is possible to obtain with the available resources.The method also permits obtaining the best order of replacement of thecircuits i.e. the order permitting allocation of the above-mentionedseries of circuits while avoiding collisions.

To be able to define which would be the better of two circuits thatsatisfy the same requirement, a ‘cost’ definition of a circuit is used.In accordance with a representation well known to those skilled in theart the network can be represented by a graph with nodes connectedtogether by means of links. With each link are associated parametersthat define the cost of use of that link. These parameters are notdescribed in detail as they depend on the routing criterion it is wishedto apply and not on the method in accordance with the present inventionwhich can be applied to any routing criterion as mentioned above.

It should also be considered that the assignment of cost parameters tothe links of a graph representing a network is a procedure well known tothose skilled in the art and needs no further explanation. The onlyassumption made here for the sake of simplicity is that the chosenrouting algorithm, whatever it might be, will use a cost function thatassigns to each specific routing circuit a single cost. For example, if,as usual, a cost is given to each link, the cost of the circuit can bethe sum of the costs of the links it traverses.

Since it is a requirement of the network to satisfy a series ofservices, a traffic matrix is also defined. As well known to thoseskilled in the art, this matrix is a series of demands R_(i) thatindicate source, destination and characteristics of the traffic. For thesake of simplicity, the traffic matrix is considered unvarying in thisprocedure.

A series of circuits that satisfy the traffic matrix is called a‘routing’ with the circuit C_(i-th) satisfying the demand R_(i-th) withi=1, . . . n.

The series of all the CA_(i) (i=1, . . . , n) circuits already existingon the network is the ‘actual routing’ i.e. the routing in use in thenetwork at the time.

The desired new routing i.e. the ‘target routing’ will have circuitsCT_(i) that also satisfy the R_(i) demands. As mentioned above, thetarget routing is calculated off-line by a simulator in accordance withwell-known procedures that will not be described in detail. Differentlyfrom prior art methods, the feasible target routing need not be reallyobtainable here.

‘Feasible routing’ is defined here as that routing that satisfies thetraffic matrix and can be obtained from the actual routing (or, as seenbelow, from another feasible routing) by moving one circuit at a timewith the available network resources.

The cost difference between circuits A and B is the cost of the circuitpart A that is not superimposed on circuit B. This cost is used toevaluate how best a circuit approximates another. It is clear that thecost is always non-negative and that the cost difference between twoidentical circuits is zero.

If circuit B is part of the target routing and circuit A is part of afeasible routing and A and B satisfy the same demand R_(i) in thetraffic matrix, account will be taken only of the resources used by Aand not by B. No discount is granted for non-use of B resources.

Given a feasible routing G and a target routing, we define best circuitas the circuit offering the best satisfaction of the demand R_(i) in G,i (designated briefly by bs(i,G), from the initials of ‘bestsatisfaction’) the circuit which simultaneously:

-   (a) Satisfies the demand R_(i);-   (b) Can be allocated together with all the circuits of G already    existing and associated with the other demands; and-   (c) Minimizes the cost difference with the optimal circuit CT_(i),    i.e. a corresponding circuit of the target routing.

A feasible routing can be obtained by moving one circuit at a time ontoanother feasible routing.

Briefly, the method in accordance with the present invention calls forrealizing a series of movements starting from the actual routing througha series of feasible routings towards the target routing and stoppingwhen all the demands R_(i) have been considered. The choice of the bestmove, which is the move permitting the best approximation of the targetrouting for a specific demand, is done by minimizing the cost of theassociated circuits.

FIG. 2 shows a possible flow chart of the method. To implement themethod, two iterative processings nested one in the other are realized.The inner iteration finds the best circuit to modify (i.e. the besttransition between a feasible routing and another feasible routing tothe desired target routing by moving a single circuit). In greaterdetail, the inner iteration removes one circuit at a time and tries toreplace it with the corresponding circuit in the target routing. This isdone by means of the search for the least cost path where the cost isthe difference from the circuit in the target routing. In this manner, asolution is always available since even in case of lack of resources, atthe worst the resources used for the removed circuits are madeavailable. The circuit chosen will be the one using resources of lessercost in addition to those used by the circuit in the target routing.

But the outer iteration moves a circuit at a time and repeats the inneriteration until all the circuits have been processed.

A pseudo code implementing the flow chart in accordance with the methodmight be the following:

Initialization

Set FTR on actual routing

Mark all the demands of the traffic matrix as not processed

Declare a sequence of empty re-routing

Interaction

-   -   Repeat    -   Repeat    -   Calculates bs(i,FTR): in the FTR routing, removes the circuit        CI_(i) that satisfies R_(i) and then calculates for R_(i) the        circuit with minimal cost difference from CT_(i);    -   Until all the R_(i) demands not yet processed have been        considered    -   Seeks the demand or series of demands {R_(i)} having minimal        cost difference        bs(i,FTR)−CT_(i)

For these demands {R_(i)}:

-   -   Replace with bs(i,FTR) in FTR    -   Suspend the circuit bs(i,FTR) from the re-routing sequence    -   Mark the demand R_(i) as processed

Until all the demands R_(i) of the traffic matrix are marked asprocessed

Results

FTR is the feasible target routing sought

Start the re-routing sequence . . .

Once the strategy has been calculated off-line on the network simulatorthe strategy is changed (in the form of order of the circuits to bemoved and their new routing) to the conventional network re-routingprocedures that will perform the calculated re-routing steps.

Essentially, this method produces a feasible target routing and asequence of movements for reaching it starting from the actual routing,by means of a re-routing sequence of a single circuit at a time and withnot more than one re-routing per circuit.

It should also be considered that if the resulting feasible targetrouting is different from the desired target routing it is possible thata second change through the method using the feasible target routing asnew actual routing would lead to obtaining better results i.e. closer tothe desired target routing.

The difference between the feasible target routing and the desiredtarget routing (sum of the differences on all the circuits) is asignificant parameter for deciding whether application of the method isuseful on a given network with a given actual routing.

In any case, it must be noted that even a small change could enable moreimportant changes in a subsequent passage of the method.

To better clarify the concepts of the method in accordance with thepresent invention a simple operational example is given below.

Assume having a network like the one shown diagrammatically in FIG. 3.The ‘space’ of the possible connection circuits between the end pointsA-B-C and D-E-F is the one inside the perimeter of the dotted figure.

Let the traffic matrix be:

$\begin{matrix}R_{1} & {A\text{-}D} \\R_{2} & {B\text{-}E} \\R_{3} & {C\text{-}F}\end{matrix}\quad$and the actual routing that satisfies this matrix be that shown in FIG.4.

FIG. 5 shows diagrammatically a new situation in which new resources Xare added to the network of FIG. 4. It would therefore be preferable tochange to the more efficient routing shown in FIG. 6 obtained bystraightening the circuits that satisfy the demands R₁, R₂, and R₃ ofthe traffic matrix. In other words, FIG. 6 shows the target routing.

The problem is therefore to change from the situation of FIG. 4 to thatof FIG. 6.

By applying the method of the present invention, the circuit thatsatisfies the demand R₁ in the actual routing is removed and the circuitthat has the minimal cost difference from the target circuit is sought.In the example, it is seen that at the moment (FIG. 5) the only circuitthat can replace the present circuit A-D that is removed is identical tothe circuit removed. In other words, the cost difference is equal to thetotal cost of the removed circuit A-D.

Then the circuit that satisfies the demand R₂ in the actual routing isremoved and the circuit that has the minimal cost difference from thetarget circuit is sought. The situation is identical to that faced forthe demand R₁; the only circuit that can replace the present circuit B-Eis identical to it and the cost difference is equal to the total cost ofthe removed circuit B-E.

The circuit of R₃ is removed last. Thanks to the new resources in X itis possible to replace the actual routing circuit with the targetrouting circuit and the cost difference is zero. The demand R₃ is markedas ‘already considered’ and the new circuit C-F is inserted in there-routing list. The routing to be developed therefore becomes that ofFIG. 7.

Since the demands are not all marked as considered, the external loopreturns to the start of the internal loop.

The internal loop starts over from the beginning with the new routing tobe developed that is assumed equal to the routing with the new circuitfor R₃ as shown in FIG. 7.

The circuit that satisfies the demand R₁ in the routing of FIG. 7 istherefore removed again. But the situation for R₁ is still the same asthe preceding one (the only circuit that can replace the present one isidentical to it and the cost difference is equal to the total cost ofthe removed circuit).

In the next step of the internal loop, in the routing of FIG. 7 thecircuit of R₂ is then removed. Thanks to the fact that the circuit thatsatisfies R₃ is now that of FIG. 7, it is possible to replace thecircuit of R₂ in the routing of FIG. 7 with the circuit of the targetrouting and the cost difference is zero. The demand R₂ is thereforemarked ‘already considered’ and the new circuit B-E is inserted in there-routing list.

It is thus possible to restart the loop with the new routing to bedeveloped that is assumed equal to the routing of FIG. 8.

The circuit that satisfies the demand R₁ in the routing of FIG. 8 istherefore removed again. Now the circuit can be replaced with that ofthe target routing. Therefore the demand R₁ is also marked ‘alreadyconsidered’ and the new circuit A-D is inserted in the re-routing list.At this point all the demands are exhausted and thus the external loopis also terminated.

Since the feasible target routing that was found coincides with thedesired target routing is not necessary, no new use of the method isnecessary for seeking to improve the result further.

Diagrammatically, the calculated re-routing list or sequence that willbe changed to the known re-routing routine of the network will be of thefollowing type:

First: ‘straightens’ C-F that satisfies R3;

Second: ‘straightens’ B-E that satisfies R2; and

Third: ‘straightens’ A-D that satisfies R1.

This list is changed and performed by the re-routing procedure of thenetwork.

In a less fortunate case in which the feasible target re-routing doesnot coincide with the desired target re-routing, some data couldadvantageously be supplied for deciding whether the re-routing sequenceis to be applied or not. For example, the more significant data can bethe overall distance cost between the actual routing and the feasibletarget re-routing (cost that we might call ‘practical improvement’) andthe overall distance cost between the feasible target re-routing and thedesired target routing, a cost that we might call ‘unobtainableimprovement’.

For example, a good practical improvement is necessary to give realbenefits. On the other hand, a poor practical improvement combined witha good ‘unobtainable’ improvement might appear as bad news but despitethis a slight practical improvement might lower the poor conditioning ofthe network and allow greater improvement in a subsequent step andtherefore counsel all the same making a new application of the method tothe feasible target routing that was obtained.

The decision as to implementation or not should be left to a humanoperator with the capability of experimenting on the simulator with thesupport of a suitable documentation of the results.

It is now clear that the predetermined purposes have been achieved bymaking available a method allowing deciding the steps to be used toreach or in any case approximate as much as possible a predeterminedtarget routing with minimal disturbance of the services in the networkand minimum risks.

As mentioned above, the overall method in accordance with the presentinvention allows freely choosing different preferred routing algorithms,it being sufficient that only the target cost function be satisfied. Ina dual manner, the method allows the routing algorithm to perform thechoices for re-routing a large series of circuits in a controlledmanner.

Naturally the above description of an embodiment applying the innovativeprinciples of the present invention is given by way of non-limitingexample of said principles within the scope of the exclusive rightclaimed here.

1. A method for reconfiguring a telecommunications transport networkafter addition or removal of a network resource, the method comprising:identifying a sequence of single circuit movements to modify the networkfrom a set of n actual circuits CA_(i) (i=1, . . . , n), each satisfyinga corresponding demand R_(i) to a set of feasible intermediate circuitsCI_(i) which continue to satisfy the demands R_(i) and which bestapproximate a series of target circuits CT_(i), comprising: (a)initializing, at a network simulator, the circuit set CI to CA; (b) foreach demand R_(i) still to be processed (i) calculating, at the networksimulator, one or more candidate replacement circuits CI_(i), eachcandidate replacement circuit CI_(i) satisfying the demand R_(i) andhaving a lower cost difference with respect to the corresponding targetcircuit CT_(i) than the current circuit CI_(i) satisfying the demandR_(i); (ii) replacing, at the network simulator, the current circuitCI_(i) with the candidate replacement circuit CI_(i) having the leastcost difference; and (iii) marking, at the network simulator, the demandR_(i) as having been processed; and (c) identifying, at the networksimulator, the sequence of single circuit movements with which circuitsCI_(i) were replaced as the series of single circuit movements to modifythe network.
 2. The method of claim 1 wherein each circuit comprises oneor more legs connecting two or more nodes, and wherein calculating thecost difference of a candidate replacement circuit CI_(i) with respectto the corresponding target circuit CT_(i) comprises summing the costsof the legs of the circuit CI_(i) that do not overlap with the legs ofthe target circuit CT_(i).
 3. The method of claim 2 wherein calculatingthe cost difference further comprises excluding a cost associated withan unused leg of the target circuit CT_(i).
 4. The method of claim 1wherein the cost of a circuit is the sum of the cost of each circuitleg.
 5. The method of claim 1 further comprising, after processing alldemands R_(i), determining whether to take the sequence with whichcircuits CI_(i) have been replaced as the series of single circuitmovements to modify the network, or whether to repeat step (b) using thecurrent set of feasible intermediate circuits CI_(i).
 6. The method ofclaim 5 wherein the determination is made based on the overalldifference in cost between the CA circuits and the CI circuits.
 7. Themethod of claim 5 wherein the determination is made based on the overalldifference in cost between the CI circuits and the CT circuits.
 8. Themethod of claim 1 further comprising providing the identified sequenceof single circuit movements to a network manager for implementation onthe network.
 9. The method of claim 8 further comprising performing theidentified sequence of single circuit movements on a network by thenetwork manager.
 10. A telecommunications transport network comprising:a plurality of circuits that satisfy a corresponding plurality ofdemands R; and a network simulator operative to reconfigure thetelecommunications transport network after addition or removal of anetwork resource by identifying a sequence of single circuit movementsto modify the network by: (a) initializing a circuit set CI to CA,wherein CA comprises a set of n actual circuits CA_(i)(i=1, . . . , n),each satisfying a corresponding demand R_(i), and wherein CI comprises aset of feasible intermediate circuits CI_(i) which continue to satisfythe demands R_(i) and which best approximate a series of target circuitsCT_(i); (b) for each demand R_(i) still to be processed (i) calculatingone or more candidate replacement circuits CI_(i), each candidatereplacement circuit CI_(i) satisfying the demand R_(i) and having alower cost difference with respect to the corresponding target circuitCT_(i) than the current circuit CI_(i) satisfying the demand R_(i); (ii)replacing the current circuit CI_(i) with the candidate replacementcircuit CI_(i) having the least cost difference; and (iii) marking thedemand R_(i) as having been processed; and (c) identifying the sequenceof single circuit movements with which circuits CI_(i) were replaced asthe series of single circuit movements to modify the network.